[111.12] The Princeton Mark IV Pulsar Observing System

Highly accurate timing of radio millisecond pulsars has
numerous applications, including experimental tests of
general relativity and cosmology.
Great improvements over existing timing measurements can be achieved by
removing the dispersive effects of the interstellar medium in a
phase-coherent manner. Only recently have computer speeds approached
levels allowing the routine use of this
method over bandwidths of several MHz or more.

We have designed the Princeton Mark IV system to implement
coherent dedispersion in software.
A fast analog-to-digital converter samples
quadrature components of the received voltages in two orthogonal polarization
channels, allowing 10\,MHz bandwidth with 2-bit sampling or 5\,MHz
bandwidth with 4-bit sampling, and producing a continuous throughput
of 10\,MB/s. The resulting data stream passes through a SPARC 20 computer and
onto DLT 7000 tape drives and/or a 108\,GB disk array. Data
analysis is performed off-line by one of several fast (1.25\,Gflop) parallel processors.
The effects of dispersion are removed by convolving the
data time-series with the inverse of the interstellar medium ``chirp
function.'' Cross-products are formed from the dedispersed
signals, and then can be folded modulo the pulse period. In this fashion
we obtain full Stokes parameters for every observation.
Narrow-band radio-frequency interference can be
effectively excised as part of the coherent signal-processing task,
while time segments contaminated with broad-band noise are dropped from
the analysis.

We present the results of several months of observations at the 76\,m
Lovell telescope at Jodrell Bank, U. K. \,\, Pulse profiles with full
polarization information and flux calibration are given for a number
of millisecond pulsars. A concentrated campaign to observe the
double-neutron-star binary PSR B1534+12 has resulted in a
greatly improved measurement of the orbital period derivative, and
hence a strong test of the predictions of general relativity.